Improve server performance despite burden of shadow copies

When you make shadow copies of shared folders, your system performance suffers. Here's some advice on how to reduce the performance hit.

Shadow copies of shared folders carry a performance penalty. How severe that penalty is, and what you should do about it, depends on the load on your system.

If there is a problem you can do something to improve performance: Modify your storage architecture.

Shadow copies use a copy-on-write algorithm to update the volume image. This incurs a performance cost since information has to be written twice to disk, increasing the I/O load on the system.

In the case of a heavily loaded server, the cost of the additional write can noticeably affect system performance. However, you can reduce the performance penalty by dedicating a separate hard disk to the shadow copy. This avoids extra disk head seeks incurred when the system writes the shadow copy to the same disk as the main copy.

For lightly loaded servers, in terms of I/O bandwidth and disk space, the cost of shadow copies generally isn't even noticeable and Microsoft recommends leaving the shadow copy service storage in its default configuration.

If and when shadow copy overhead for shared folders becomes a problem depends on the system configuration and the details of the load on the system. For this reason, Microsoft says it is impossible to determine the precise amount of overhead. If you suspect a problem, use the disk analysis utilities in Windows or other tools to examine the performance of the disk and then decide if you need to take remedial action.

Microsoft discusses the issues briefly here: http://www.microsoft.com/windowsserver2003/techinfo/overview/scrfaq.mspx.

 


Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.


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This was first published in October 2005

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